Shadowfang Keep 18-21?

I understand what you're wanting to accomplish by shortening the level ranges of the mobs in an instance, and SFK is a good example of where it needs to be done. But I don't think you looked at how people generally level when you considered this.

At 18-21, Alliance is running Deadmines. Horde is running Wailing Caverns. Now, Wailing Caverns has a pretty widespread level range too, but because of it's location in the Barrens (most common Horde leveling), it's easy to find groups for and finish, and if anything, this is the instance that should be tuned for 18-21 which basically just means lowering the levels of Verdan the Everliving and Mutanus the Devourer, as these are only two things that give even PuG WC groups any kind of a hard time.

Shadowfang Keep however, (and I admit part of this is because of the end level range) is not done by Horde until 21-24, and I doubt you're going to have a stream of Alliance wanting to run all the way from Menethil Harbor, through the Arathi Highlands, into Hillsbrad Foothills, and then into Silverpine Forest before 21, not when they can level in Westfall and get a group for Deadmines. Also, because Silverpine Forest is poorly laid out as a 12-20 questing zone with 24 elite Sons of Arugal wandering around it, it's not a very popular spot to level, especially since the introduction of Ghostlands.

Therefore, I suggest to you, instead of going with the low end of the current SFK, tune it for 21-24 instead. Players who run SFK at this level range, will find themselves geared and leveled enough that they won't be running into The Stockades and Blackfathom Deep unprepared.